The present invention relates to a method and apparatus for testing geophones and particular stations of geophones. The term "geophone stations" is used to describe strings of geophones that are planted on the earth's surface and the cables used to connect them to a recording system. Further, the term"geophone stations" normally refers to the geophone strings and their connecting cables that are connected to a single recording channel on the acquisition equipment. Conventional land seismic crews utilize 48 to 96 channels of recording or acquisition equipment. Thus, the normal land crew would have 48 to 96 geophone stations with several strings of geophones per station.
When geophones are planted on the earth'surface, some are inclined at an angle which prevents the movable coil from correctly responding to the earth's vibration while others may be suspended in air and not in contact with the earth. Also, due to the rough handling of geophones and their associated cables, physical damage is done to the goephones which prevents them from responding properly to the earth's vibrations. Rough handling can also cause the connecting cables to break and introduce an open circuit in the system.
In the past, various means have been suggested for testing geophones and their associated cables and recording systems for proper operations. One such method is the creation of vibration in the earth as by stomping on the earth or dropping a weight adjacent the geophones and recording their response. While these methods have some success it is inconvenient to coordinate between the personnel producing the vibrations and those recording the data.
In U.S. Pat. No. 3,858,169, there is suggested a system for testing geophones and their associated circuits by introducing a current impulse to the geophones. The geophone, when responding to vibrations in the earth produced by seismic waves, is responding primarily to the velocity incident on the geophone case. Thus, it is apparent that if one could introduce a velocity impulse to the geophone by some external means, one could analyze the response of the geophones and their associated circuitry. This would then permit one to determine whether some geophones were inoperative due to physical damage or improper planting on the earth's surface. The patent suggests that a velocity impulse could be applied to the geophones by utilizing very short current impulse having a duration of 1 to 2 milliseconds. The patent particularly stresses that the impulse must be short enough so that little or no physical motion will be imparted in the moving coil of the geophone. After the application of the impulse the response of the geophones is recorded on the normal data acquisition system which provides a record of the response of the geophone and associated circuits. It is claimed this recording can be used to deconvolve subsequent seismic data to remove the effect of the geophones and their associated circuitry from the recorded data.
While the above described system can theoretically operate in practice, it is extremely difficult to supply the accurate current pulse required. There is also some question as to the correctness of their claims. The cables used in conventional geophone circuits can be several from hundreds of feet to tens of thousands of feet long and thus the capacitance of the cables alone prevents the application of a very short current pulse to all of the geophones. As the patent points out, if there is any motion imparted to the coil the system is no longer responding to a velocity impulse but rather a displacement impulse. A displacement impulse would produce an erroneous geophone response and if it were used to deconvolve seismic data, the interpretation of the data would also be in error.